1. Field of the Invention
The present invention relates to liquid crystal display devices. More particularly it relates to liquid crystal display devices preventing light leakage in peripheral portions.
2. Discussion of the Related Art
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules to produce an image. Liquid crystal molecules have a definite orientational alignment as a result of their long, thin shapes. That orientational alignment can be controlled by an applied electric field. In other words, as an applied electric field changes, so does the alignment of the liquid crystal molecules. Due to the optical anisotropy, the refraction of incident light depends on the orientation of the liquid crystal molecules. Thus, by properly controlling an applied electric field a desired image can be produced.
Of the different types of known LCDs, active matrix LCDs (AM-LCDs), which have thin film transistors and pixel electrodes arranged in a matrix form, are the subject of significant research and development because of their high resolution and superiority in displaying moving images.
LCD devices have wide application in office automation (OA) equipment and video units because they are light and thin, and have low power consumption characteristics. The typical liquid crystal display (LCD) panel has an upper substrate, a lower substrate and a liquid crystal layer interposed therebetween. The upper substrate, commonly referred to as a color filter substrate, usually includes a common electrode and color filters. The lower substrate, commonly referred to as an array substrate, includes switching elements, such as thin film transistors (TFTs), and pixel electrodes.
As previously described, LCD device operation is based on the principle that the alignment direction of the liquid crystal molecules is dependent upon an electric field applied between the common electrode and the pixel electrode. Thus, the alignment direction of the liquid crystal molecules is controlled by the application of an electric field to the liquid crystal layer. When the alignment direction of the liquid crystal molecules is properly adjusted, incident light is refracted along the alignment direction to display image data. The liquid crystal molecules function as an optical modulation element having variable optical characteristics that depend upon polarity of the applied voltage.
FIG. 1 is a schematic cross-sectional view illustrating a conventional LCD cell in an active matrix LCD. As shown, the LCD cell 20 has lower and upper substrates 2 and 4 and a liquid crystal (LC) layer 10 interposed therebetween. The lower substrate 2 has a thin film transistor (TFT) “T” as a switching element that switches a voltage that changes the orientation of the LC molecules. The lower substrate 2 also includes a pixel electrode 14 that is used to apply an electric field across the LC layer 10 in response to signals applied to the TFT “T.” The upper substrate 4 has a color filter 8 for producing a color, and a black matrix 7 shielding light in non-display regions and preventing the thin film transistor from light irradiation. Furthermore, the upper substrate 4 includes a common electrode 9 on the color filter 8 and black matrix 7. The common electrode 9 serves as an electrode that produces the electric field across the LC layer (with the assistance of the pixel electrode 14). The pixel electrode 14 is arranged over a pixel portion “P,” i.e., a display region. A lower orientation layer 11 that serves to align the liquid crystal molecules for selectively transmitting light is disposed on the pixel electrode 14 and over the TFT “T,” while an upper orientation layer 5 that serves to align the liquid crystal molecules with the lower orientation layer 11 is on the common electrode 9. Further, to prevent leakage of the LC layer 10, a pair of substrates 2 and 4 are sealed by a sealant 6.
The lower substrate needs several more processes than the upper substrate, such as a deposition process and a photolithography process. Since the deposition and photolithography processes are carried out several times when forming the array substrate, several steps are formed in an array matrix type on the lower substrate.
FIG. 2 is a schematic partial plan view showing a display region and non-display regions of the liquid crystal display of FIG. 1. In FIG. 2, the liquid crystal display cell 20 is widely divided into the display region 40 in which images are displayed, and the non-display regions 30 in which alignment keys (not shown) and shorting bars (not shown) are disposed. The alignment keys are points to align the upper substrate to the lower substrate, and the shorting bars serve in the circuit test. The non-display regions 30 are detached in later steps after the circuit test and attaching the upper substrate to the lower substrate. In the display region 40, a plurality of gate lines 42 are arranged in a transverse direction and a plurality of data lines 44 are arranged perpendicular to the plurality of gate lines 42. In each crossover point of the gate lines 42 and the data lines 44, a thin film transistor (TFT) “T” is disposed in a matrix type. A plurality of pixel electrodes 46 are placed in pixel regions defined by the gate lines 42 and the data lines 44. Further, the plurality of pixel electrodes 46 are divided into first pixel electrodes 46a disposed in peripheral portions of the display region 40, and second pixel electrodes 46b disposed in an inner portion of the display region 40.
Still referring to FIG. 2, an arrow “RD” represents an exemplary rubbing direction when the lower orientation layer 11 of FIG. 1 is rubbed for aligning the liquid crystal molecules. At this time, the periphery such like areas “L1” is irregularly rubbed due to the steps of the array matrix structure. Especially, when the rubbing direction is from bottom to top as shown in FIG. 2, the rubbing irregularity exceedingly occurs in both right and left areas such like the area “L1.” Further, this rubbing irregularity causes light leakage in the periphery of the liquid crystal display when a driving voltage is applied to the pixel electrodes 46.
FIG. 3 is a cross-sectional view taken along the line III-III of FIG. 2 and illustrates a peripheral portion of the display region. In FIG. 3, the lower substrate 2 includes the first pixel electrode 46a and the data lines 44 which are disposed over the substrate 1 and separated from each other. The lower orientation layer 11 is disposed on the first pixel electrode 46a and over the data line 44. The upper substrate 4 includes the black matrix 7 and color filter 8 on the substrate 1. The common electrode 9 is disposed on the black matrix 7 and color filter 8, and the upper orientation layer 5 is disposed on the common electrode 9. The black matrix 7 corresponds to the non-pixel region in which the data line 44 is arranged, while the color filter 8 corresponds to the pixel region that is the display region. Liquid crystal molecules 15 are disposed between the lower substrate 2 and the upper substrate 4, thereby forming a liquid crystal layer 10. As a material for the upper and lower orientation layers 5 and 11, polyimide is mainly used. Polyimide aligns the liquid crystal molecules so that they are parallel to the substrate 1.
In general, the upper and lower orientation layers 5 and 11 are rubbed in one direction to align the liquid crystal molecules 15 and make the liquid crystal molecules 15 have pretilt angle. In the rubbing process, a rubbing fabric rolled onto a roller is widely used and the upper and lower orientation layers 5 and 11 are rubbed to have a plurality of minute grooves on their surface using this rubbing fabric. Therefore, the surfaces of the orientation layers are rubbed in a uniform direction. However, the peripheral portions of the liquid crystal display, such as a region “R” of FIG. 3, are irregularly rubbed rather than an inner portion. Especially, this rubbing irregularity easily occurs in the lower substrate 2 due to the fact that the lower substrate 2 includes several array matrix structures and has several steps caused by the array matrix elements. Especially, because of the steps between the display region 40 and the non-display regions 30 (in FIG. 2), the region “R” of FIG. 3 in the first pixel electrode 46a is not properly rubbed. Thereby, the liquid crystal molecules 15 disposed within the region “R” of the first pixel electrode 46a are not properly arranged when the voltage is applied to the pixel electrode, thereby giving rise to light leakage. The irregularities of the surface of the region “R” (i.e., the irregularly rubbed area) depend on the rubbing direction of the orientation layer.
As described above, light leakage occurring in the periphery of the liquid crystal display deteriorates and decreases the display quality of the liquid crystal display device.